#101 - lowercased keywords

[krzysztofrewak]

I found out that NULL and TRUE and FALSE were still allowed to be written with uppercase. This PR is continuation of #104 and should finally close it. It's connected to #101.

Additionally I added some optimization elements. Codestyle should be cacheable and parallel runnable by default from now. If anyone would disable it for any reasons, there're some methods for that:

$config->withoutCache();
$config->withoutParallelRun();

There's also a way to tune parallel run:

$config->withCustomParallelRunConfig(new \PhpCsFixer\Runner\Parallel\ParallelConfig(4, 20));

Additionally I added fixer for PHPUnit annotations as old way is deprecated.